RFID tags (Radio Frequency-IDentification TAG) having an antenna coil and a control device such as an IC circuit, or data carriers (child) such as non-contact type IC cards are widely used in various fields to serve as communication devices communicating by electromagnetic wave, in which the communication devices communicate with a read/write device (reading device or a read/write device) or the like which serve as a parent.
The RFID tag is, for example, used for purposes such as product management, and the IC card is used for purposes such as transit passes, commuting passes and cash cards. The data carriers can be classified into a portable type and a stationary type being attached to a device, a machine, or various components, in which both types use high frequency electromagnetic wave in a wireless communication area to perform non-contact sending/receiving (communication) of data between a read/write device (or a reading device).
The data carrier itself normally has no electric power source such as a battery for operation, but has a portion of electromagnetic wave sent by the read/write device serving as an electric power source.
FIG. 3 is an explanatory view showing a data carrier structure, and FIG. 4 is a block diagram thereof. As shown in FIG. 3(a), an RFID tag 1a, a conventional communication device serving as a data carrier, comprises a disk shape antenna coil 2a having conductive wires wound to form an air-core coil, and a semiconductor IC chip 4 having an IC circuit connected to both ends of the antenna coil 2a. 
As shown in FIG. 4, the semiconductor IC chip 4 comprises: a sending/receiving terminal 4c; a CPU (central processing unit) 4a; a memory 4b having a writable nonvolatile memorizing element; and a capacitor 4d for power storage means. The antenna coil 2a and the semiconductor IC chip 4 are formed into a thin disk-shape or a card shape by being molded into a united body with a resin material, or formed into a sealed shape by being laminate-processed for protection from the external
In explaining a sending/receiving method of the RFID tag 1a with reference to FIG. 4, a read/write device (not shown) first sends an electromagnetic wave for calling the RFID tag 1a and for transmitting electric power. The RFID tag 1a then receives the electromagnetic wave by a tuning effect of the antenna coil 2a and the sending/receiving terminal 4c, and stores the electric power inside the capacitor 4d. Subsequently, the RFID tag 1a becomes operational to then send an electromagnetic wave from the read/write terminal to the RFID tag 1a for a readout purpose in the following step.
Then, the electromagnetic wave enters from the antenna coil 2a of the RFID tag 1a to the CPU 4a via the sending/receiving terminal 4c; then, the CPU 4a reads out necessary information from the memory 4b in accordance to the entered electromagnetic wave; and then, the information in a form of an electromagnetic wave is sent from the sending/receiving terminal 4c to the read/write device via the antenna coil 2a. The above method is also employed when writing data from the read/write device to the memory 4b of the RFID tag 1a. It should now be noted that the above consecutive steps are relatively performed instantaneously.
FIG. 3(b) is an explanatory view showing a relation between the antenna coil 2a and an electromagnetic wave. An electromagnetic wave used in communication generally can be referred as an electric field and a magnetic field simultaneously propagating while being normal to each other, in which the communication with the electromagnetic wave performs sending/receiving with use of an electrical current (high frequency electrical current) flowing in the antenna coil 2a by interlinking the magnetic field and the antenna coil 2a. 
For example, when an electromagnetic wave is sent from the antenna coil 2a, a high frequency electrical current flowing at the antenna coil 2a allows a high frequency magnetic field component H (as illustrated in FIG. 3(b)) to be distributed as a loop (magnetic flux loop) passing through a center of the antenna coil 2a, and the read/write device is able to receive information from the RFID tag 1a by placing an antenna coil of the read/write device in an area of the magnetic flux. Likewise, when an electromagnetic wave is sent from the read/write device, the magnetic field component H is distributed as illustrated around the antenna coil 2a of the RFID tag 1a to allow the antenna coil 2a to receive the electromagnetic wave.
Communication distance, that is, the communicable distance between the RFID tag 1a and the read/write device is normally a couple millimeters to a couple centimeters. For example, an automatic gate for trains hardly has any trouble in communicable distance since the data carrier such as a commuting pass could be read out by inserting and skimming the pass closely upon the reading portion arranged inside the automatic gate.
However, in a case such as attaching a data carrier such as an RFID tag upon a product for product management, the use of the data carrier would be restricted if the communication distance is short. Other cases may also require high communication directivity in a particular direction depending on the type of management. Therefore, various methods of extending communication distance and improving directivity for the data carrier have been proposed in the past.
The communication distance using electromagnetic wave requires a sending antenna coil and a receiving antenna coil to be arranged within an area of the magnetic field for maintaining a communicable magnetic flux density level. Although the size of the communicable area for the magnetic field, that is, the communication distance relies on a power level of the sending side, the directivity of the antenna coil of the RFID tag is an influential factor when power level of the sending side and the receiving side are equal.
In a case such as attaching an RFID tag to a metal surface, the alternating magnetic field produced by an electromagnetic wave purposed for tag sending/receiving creates an eddy current inside the metal. The eddy current produces a magnetic flux opposing to a magnetic flux purposed for sending/receiving and thereby attenuates the magnetic flux purposed for sending/receiving and causes difficulty in sending/receiving. A material for attenuating the initial magnetic flux will hereinafter be referred to as “conductive material”.
A known method in attaching a member made from a conductive material to an RFID has a magnetic material arranged between the RFID tag and the attaching surface of the conductive material for passing a sending/receiving magnetic flux thereto so that the magnetic flux can enter the conductive material and restrain the creation of an eddy current.
Furthermore, a method, which has a sheet-like amorphous magnetic material with a higher magnetic permeability (hereinafter referred as “sheet-like magnetic material”) serving as the magnetic material, is proposed as a method using a thin sheet for efficiently bypassing a magnetic flux without increasing much space (as shown in Japanese Patent Publication Hei 8-79127).
With the foregoing conventional example shown in Japanese Patent Publication Hei 8-79127, the sheet-like magnetic material is arranged across the entire surface of the sending/receiving antenna coil of the RFID tag. However, through the findings of the inventors of this invention, even if the sending/receiving sensitivity of the RFID tag is slightly improved compared to a situation where the sheet-like magnetic material is not arranged entirely across the antenna coil, there is not much difference from a practical aspect; in some cases, a closed loop which passes through the sheet-like magnetic material is created to adversely result in decline of sensitivity.
In a method proposed in Japanese Patent Publication No.2000-48152, the RFID tag 1a is comprised of a disk-shaped antenna coil 2a having an insulated wire such as an enamel wire wound around a circular air-core coil, and a semiconductor IC chip 4 connected to both ends of the antenna coil 2a, in which the antenna coil 2a has a thin sheet-like magnetic material constituted of an amorphous sheet inserted therethrough. The sheet-like magnetic material has a surrounding thereof covered with an insulated sheet to prevent the sharp edges of the sheet-like magnetic material from damaging an insulated covering formed at the surface of the antenna coil 2a. 
The sheet-like magnetic material is chosen to have a width smaller than the diameter of a hollow portion of the antenna coil 2a for being inserted into the antenna coil 2a, and after the sheet-like magnetic material is inserted into the antenna coil 2a, the sheet-like magnetic material and the antenna coil 2a are pressingly shaped into a flat form.
Since the sheet-like magnetic material has an exceedingly low magnetic resistance compared to that of the air, the magnetic flux interlinking with the antenna coil 2a easily extends in a longitudinal direction of the sheet-like magnetic material and is distributed into the air as a magnetic flux loop passing through the tip-end portion of the antenna coil 2a. Accordingly, communication distance is extended mainly in the longitudinal direction of the sheet-like magnetic material, and communication directivity is also heightened in thus direction.
However, with the foregoing method proposed in Japanese Patent Publication No.2000-48152, manufacturing is difficult and laborious since the sheet-like magnetic material is required to be inserted to the antenna coil 2a. There is also a risk of damaging the insulated covering of the antenna coil 2a when the sheet-like magnetic material and the antenna coil 2a are pressingly shaped into a flat form after the sheet-like magnetic material is inserted into the antenna coil 2a, and therefore, a means for preventing such risk is required.
The RFID tag 1a having the antenna coil 2a and the semiconductor ID chip 4 integrally sealed with resin is generally produced in mass numbers, and standard products with short communication distance are being sold in the market inexpensively. However, with the foregoing method, it would be necessary for the standard products to be manufactured in another different process since the sheet-like magnetic material 8 is required to be inserted into the antenna coil 2a. 